The velum interpositum revisited and redefined

Introduction: Descriptions of the velum interpositum (VI) are typically brief and lacking detail in most neuroanatomical and neurosurgical texts. As this structure may be involved clinically or encountered surgically, the present study seemed warranted. Materials and Methods: Twenty-adult (10 male and 10 female) formalin fixed and fresh cadaveric brains underwent a detailed dissection of the VI via an interhemispheric transcollosal approach. Observations were made of the attachment sites and continuation of the VI. Measurements were made of its length and width at its anterior, midportion, and posterior parts. Results: The VI extended laterally over the thalami tobecome continuous with the choroid plexus of the lateral ventricles. At a point along the thalami where the choroid plexus was found, the VI became "tacked" down and thus continuous with the choroid plexus subependymally. No specimen exhibited a separate choroid plexus of the third ventricle. In each, the choroid plexus of the lateral and third ventricles were the same tissue layer, all arising from the VI. This structure was adherent to but not fused to the deep surface of the fornix. The VI was also not fused to the pineal gland or habenula commissure but simply covered these structures. This membrane was confluent with the pia/arachnoid over the cerebellum and from the inferior surface of the parietal/occipital lobes and extended laterally into the choroid fissure. Conclusions: To our knowledge, the extent of the VI as described herein has not been reported earlier. The supratentorial choroid plexus is simply a vascular extension of the VI. There is no separate choroid plexus of the third ventricle as often described. Clear planes exist between the VI and surrounding structures such as the pineal gland. Such data may be useful to neurosurgeons who operate in this region and to clinicians who interpret imaging in the area of the VI.